Hi Peter (Csontos),
Yes, slew rate, risetime, and bandwidth will in the case of many designs extend well beyond the point of being overkill with respect to effects that may have DIRECT audible significance. The Spectral amplifiers being extreme examples in that respect. As I'll get into in a moment, though, that does not necessarily mean that the only benefit is to those who write marketing literature. But first for some quantitative perspective on these numbers:
A very conservative (i.e., safe) rule of thumb for bandwidth, that would rule out the possibility of perceptible phase shifts within the audible spectrum, and that would take into account the possibility that, as some have claimed, frequencies well above 20 kHz may be sensed by some listeners when accompanied by lower frequencies, would be to multiply 20 kHz by a factor of 10, meaning 200 kHz.
Then consider that the relation between risetime and 3db bandwidth (i.e., the frequency at which response has rolled off by 3 db) is, for a first order (6 db/octave) rolloff:
risetime = 0.35/bandwidth
where the units are chosen consistently between the two parameters (i.e., seconds and Hz, or milliseconds and kHz, or microseconds and mHz).
Based on that relation, a first order response that is down 3 db at 200 kHz corresponds to a risetime of 1.75 us (microseconds), very close to the 2 us figure you cited.
Slew rate, which is the fastest rate at which the amplifier output can change (presumably under large signal conditions), should be fast enough to provide a wide margin relative to values that might limit the other two parameters. If we consider the somewhat extreme example of a 200W amp, 200 watts into 8 ohms corresponds to 40 volts rms, which for a sine wave corresponds to 113 volts peak-to-peak. The half-period corresponding to a frequency of 200 kHz (i.e., the time required for a peak-to-peak swing at that frequency) is 2.5 us. 113 volts/2.5 us is about 45 volts/us. A slew rate of 100 volts/us, which Ralph (Atmasphere) rightly characterized as "fast," seems comfortable relative to that figure, considering especially that 200 watts at 20 kHz, much less 200 kHz, is an unlikely scenario :-)
Now consider in those contexts the specs on the four Spectral amps I referred to:
Bandwidth (3 db): 1.8 mHz
Risetime: 0.4 us
Slew rate: 600 v/us (800 v/us in one case)
Obviously way overkill relative to any conceivable effects that are DIRECTLY audible.
So why have I put the word directly in caps? Well, I would presume the designers feel that by designing ultra-fast circuitry they can avoid or minimize effects which may be audibly significant. For example, they may be able to realize the benefits of increased amounts of negative feedback while avoiding or minimizing what would normally be its adverse effects, such as transient intermodulation distortion. Or the higher speed circuitry might help to minimize crossover distortion, or the effects of unwanted energy storage in devices, etc.
So what can be said about that? Well, its an approach, and a philosophy. As is usual in audio, how good or bad the results are will depend on quality of implementation, system matching, and listener preference.
My understanding is that for the most part sonically, slew rate correlates to bass performance and rise time to treble. This is what I've been told by the tech who's worked on some of my amps.
That can probably be said to have a vaguely definable basis in situations where risetime and slew rate are slow enough to verge on having DIRECT audible significance, given as I said that risetime is inversely related to bandwidth (and thus potentially to upper treble extension), and bass frequencies are generally present at much greater amplitudes than upper treble frequencies (slew rate probably being best considered as a large signal parameter, as I said earlier). But the statement strikes me as a crude oversimplification at best, and one which does not have much relevance in situations where both parameters have comfortable margins relative to audible thresholds.
Best regards,
--Al
